As shown in FIG. 12 by way of example, an articulated robot disclosed as in U.S. Pat. No. 4,433,495 that moves along the shortest route to transport a work or the like between two points such as two machine tools set a comparatively long distance from each other, has a fixed base 1 with a horizontal upper surface, and a parallelogrammic linkage comprising a first arm 2 pivotally supported onto fixed base 1 for turning at one end thereof by a pin 11, a first auxiliary link 3 pivotally supported onto fixed base 1 for turning at one end thereof by a pin 12, and a second auxiliary link 4 pivotally joined at its opposite ends to the respective extremities of first arm 2 and first auxiliary link 3 by pins 21 and 31. First arm 2 turns in a horizontal plane between positions indicated by alternate long and short dash lines. A gear wheel 22 is fixed to the first arm coaxially with pin 21. A second arm 5 is fixed to a rotary shaft 41 rotatably supported on second auxiliary link 4 at the middle of the same. A gear wheel 42 is fixed to rotary shaft 41 so as to mesh with gear wheel 22. When first arm 2 is turned, second arm 5 turns in the same direction. Thus, first arm 2 and second arm 5 are driven by a single driving means.
As shown in FIG. 13, another known articulated robot disclosed, by way of example, in U.S. Pat. No. 4,661,040 has a fixed base 1 with a horizontal upper surface, and a first arm 2 supported for turning about a vertical axis S1 on fixed base 1. A swivel base 20 is supported on the extremity of first arm 2 so as to be able to turn about an axis S2 parallel to axis S2. A second arm 5 is supported onto swivel base 20 for turning in a vertical plane about an axis U1 perpendicular to axis S2, and a third arm 6 is joined to the extremity of second arm 5 pivotally for turning about an axis U2 parallel to axis U1. A fourth arm 7 is supported on third arm 6 so as to be able to move longitudinally and to turn about an axis R. A wrist unit 8 is connected to the extremity of fourth arm 7 for turning about an axis B perpendicular to axis R, and an end effecter 9 is held on wrist unit 8 so as to be able to turn about an axis T perpendicular to axis B.
In the former articulated robot, since the turning motion of the first arm is transmitted through the gear wheels to the second arm, the motion of the second arm is restricted by that of the first arm, and the degree of freedom of the arms of the articulated robot is restricted. Furthermore, this articulated robot requires a complex construction.
In the latter articulated robot, since second arm 5 is mounted on first arm 2, second arm 5 turns together with first arm 2 through the same angle when first arm 2 is turned. Accordingly, when second arm 5 is turned in a direction opposite to the direction in which first arm 2 is turned to move wrist unit 8 held on second arm 5 along the shortest route between two separate points, i.e., along a straight line connecting the two points, second arm 5 must turn through substantially twice the angle through which first arm 2 is turned and hence, the working speed of second arm 5 must be doubled. Therefore, second arm 5 needs a larger driving unit for driving the same. Since the moment of inertia of second arm 5 is born entirely by first arm 2, first arm 2 needs a large-capacity driving motor and a large-capacity reduction gear.
Accordingly, it is an object of the present invention to provide a robot arm mechanism for an articulated robot capable of operating at a high speed, requiring a driving mechanism having a comparatively small capacity, and not subject to restrictions on the degree of freedom.